The polymerization of vinylidene fluoride (VF.sub.2) in aqueous media is well known. See, for example, Kirk-Othmer, "Encyclopedia of Chemical Technology" 4th ed., vol. 11, page 694 (1994), which mentions persulfate salts and disuccinic acid peroxide among water soluble initiators that can be used and dialkylperoxydicarbonate among monomer soluble initiators that can be used. Polymerization of VF.sub.2 was first disclosed by Ford & Hanford in U.S. Pat. No. 2,435,537 using initiators such as benzoyl peroxide and ammonium persulfate. Subsequently, Hauptschein in U.S. Pat. No. 3,193,539 exemplified azo initiators, showing very low yield with azobisisobutyronitrile (AIBN), and Iserson in U.S. Pat. No. 3,245,971 found AIBN to be ineffective.
The polymerization of certain fluorinated monomers in liquid or supercritical carbon dioxide is disclosed, for example, in Japanese Patent Application Publications 45-003390 (1970, Fukui et al.) and 06-345824 (1994, Ezaki et al. ), Japanese Patent Publication 46-015511 (1971, Ukihashi et al.), and U.S. Pat. No. 5,496,901 (DeSimone) and U.S. Pat. No. 5,618,894 (DeSimone & Romack). These references disclose various means of initiating polymerization including the use of ionizing radiation and assorted free radical initiators. The '901 patent discloses an extensive list of initiators that can be used including several azo compounds, and states a preference for azobisisobutyronitrile (AIBN).
As shown by tests summarized below, azo compounds commonly used to initiate polymerization are found not to work well, if at all, in attempted polymerizations of VF.sub.2 in carbon dioxide. Notably, AIBN yields only a small amount of polymer.
However, azo initiators are desirable because of their predictable kinetics. As discussed by Sheppard & Kamath in Polymer Engineering and Science 19, 597 (1979), azo initiators do not undergo radical induced decomposition, and their decomposition rates are not affected by environment. Thus, it would be desirable to have an effective azo initiator for use in polymerizing VF2 in carbon dioxide.